Polyalkylene terephthalates are widely usable in various resin industries due to their excellent physical and chemical properties. Especially, the polyester is highly useful for producing synthetic filaments or fibers which are also useful in various fields. However, since the polyester per se is highly hydrophobic, the polyester filaments are also hydrophobic and not at all suitable for use as filaments exhibiting a water and moisture absorbing property.
In order to obtain polyester filaments exhibiting a hydrophilic property, attempts were made to modify the known polyester filaments by producing them from a blend of a polyester with a polyalkylene glycol (U.S. Pat. No. 3,329,557 and British Pat. No. 956,833) or from a mixture of a polyalkylene glycol with a metal salt derivative (U.S. Pat. No. 3,682,846). However, the hydrophilic property of such resultant polyester filaments was found to be not only unsatisfactory but also readily degraded when the polyester filaments were laundered. In addition, the above-mentioned modification was found to cause the resultant polyester filaments to exhibit decreased physical properties, especially decreased resistance to actinic rays and a decreased thermal resistance.
In order to eliminate the disadvantages of the above-mentioned modification, attempts were made to treat the modified polyester filaments in a hot water medium or a hot alkali aqueous solution so as to form wrinkle-shaped thin concaves on the surfaces of the filaments. However, the treated polyester filaments were still found to have an unsatisfactory level of water and moisture absorbing property. Also, such treatment resulted in a decrease in the physical properties, especially, the tensile strength, of the resultant polyester filaments.